Rhea: Saturn's Dirty Snowball Moon Rhea: Saturn's Dirty Snowball Moon By Nola Taylor Redd, Space.com Contributor | June 29, 2016 08:08pm ET MORE This raw image of Saturn's icy moon Rhea was taken on March 10, 2013 by NASA's Cassini spacecraft, and received on Earth March 10, 2013. The camera was pointing toward Rhea at approximately 174,181 miles (280,317 kilometers) away. Credit: NASA/JPL/Space Science Institute Rhea, the second largest moon of Saturn, is a dirty snowball of rock and ice. The only moon with an oxygen atmosphere, thin though it may be, Rhea is one of the most heavily cratered satellites in the solar system. Discovery and naming Rhea is one of the four moons discovered by Italian astronomer Giovanni Cassini . He spotted the icy moon on Dec. 23, 1672. It was the second moon he observed, and the third to be found in orbit around the ringed planet.

At 949 miles (1,528 kilometers) in diameter, it is the second largest moon of Saturn , but less than a third the size of the largest contender, Titan . Cassini named the four moons he discovered the Sidera Lodoicea — the Stars of Louis — after King Louis IV. The moon was first referred to numerically as Saturn V, based on its distance from the planet. It was not formally named until 1847, when John Herschel (son of astronomer William Herschel ) suggested that the satellites of Saturn be named for the Titans, the brothers and sisters of the Greek god Cronus (known to the Romans as Saturn). Rhea, the daughter of Uranus and Gaea, was the wife of Cronus. Known as the mother of the gods, she gave birth to several of the Olympian gods and goddesses, including Zeus and Hera. She was also mother to Hestia, Hades, Demeter, and Poseidon. The features on the moon are named for people and places from various creation myths.

[ Photo Gallery: Icy Rhea: Photos of Saturn's Second-Largest Moon ] On the surface Rhea is an icy body with a density 1.233 times that of liquid water, which implies that it is three-quarters ice and one-quarter rock. The highly reflective surface provides further evidence. When NASA's Cassini-Huygens spacecraft examined Rhea, measurements based on its angular motion led scientists to conclude the moon lacked an icy core. Instead, it resembles a frozen dirty snowball, with ice and rock mixed together. Temperatures on Rhea range from minus 174 degrees C (minus 281 degrees F) in the warmer, sunlit areas to minus 220 C (minus 364 F) in the shade. At these frigid temperatures, the ice on the moon behaves like rock. Rhea is one of the most heavily cratered satellites in the solar system. Regions on the moon are classified based on their cratering. Heavily cratered, brighter terrain contains impacts larger than 25 miles (40 km), while more lightly cratered regions near the poles and equatorial regions have smaller impacts.

Rhea is more heavily cratered than Saturn's other icy moons, Dione and Tethys. It also orbits farther from the planet, averaging 327,490 miles (527,040 km) away. As a result, Saturn's gravitational tug has less of an effect on the moon. Dione and Tethys both show signs of smooth plains, which may have been created when the internal warmth caused by Saturn's gravity melted and re-iced cratered regions. Another explanation is that the distant moon could simply have received more impacts. The March 2010 Rhea flyby trajectory and oxygen atmosphere distribution (as simulated by computer models). Inset: Predicted oxygen density (yellow), compared to Cassini's measurements (white) taken during the flyby. Credit: Science/AAAS Rhea also contains wispy lines, running tens to hundreds of kilometers across the planet. The Cassini craft revealed that these wisps were fractures in the surface that created canyons, some of which reached several hundred meters in height. As dark material falls from the walls, bright icy material beneath is revealed. Dione and Tethys also house similar wisps.

[ Photo Gallery: Icy Rhea: Photos of Saturn's Second-Largest Moon ] A thin atmosphere and rings that weren't A very faint oxygen atmosphere exists around Rhea, the first direct evidence of an oxygen atmosphere on a body other than Earth. The atmosphere is thin, with oxygen measuring about 5 trillion times less dense than that found on Earth. Oxygen could be released as the surface is irradiated by ions from Saturn's magnetosphere. The source of the carbon dioxide is less clear, but could be the result of similar irradiation, or from dry ice much like comets. At one point, scientists announced the possibility of three narrow bands filled with material ranging in size from pebbles to boulders. The original detection came from plasma measurements taken by Cassini, but later optical observations revealed no rings. The source of the plasma measurements remains a mystery. From maps and data Cassini obtained, scientists are trying to figure out how old features on the surface are, as well as the nature of these features.

A 2014 study in the journal Icarus showed that Rhea's Inktomi crater and the blanket of debris surrounding it have more thermal inertia (resistance to being the same temperature as the surroundings) than other areas of the moon. A separate presentation at the American Astronomical Society in 2016 suggests that Rhea (as well as several other moons) would have pelted from the debris of a generation of older moons, in its distant past. A 2015 study in Icarus looked at the relative amounts of crystalline and amorphous (less distinct) water ice, which is useful for worlds that are predominantly icy. The team studied two craters, Inktomi and Oblata, and found that most of the crystalline ice was in the center of these craters, which was expected because that's where the small body impacted the surface and caused the most disruption. Over time, crystalline ice is exposed to charged particles and it becomes less pristine. Using a known rate of ice transformation, the team estimated the Oblata crater was about 450 million years old.

More facts about Rhea Diameter: 949 miles (1,528 km) Equatorial circumference: 2,057,770 miles (3,311,665 km) Surface area: 2,834,260 square miles (7,340,701 square miles) Mass: 2.306 x 1021 kilograms Density: 1.233 grams per cubic centimeter Average orbit around Saturn: 327,505 miles (527,068 km) Closest approach: 327,177 miles (526,541 km) Farthest approach: 327,832 (527,595 km) Orbit eccentricity: 0.001 Tethys and Rhea moons - Cassini captures Saturn and its moons - Pictures - CBS News Cassini captures Saturn and its moons Next Saturn NASA's unmanned Cassini spacecraft, launched on October 15, 1997, is the fourth space probe sent to Saturn and the first to enter its orbit, studying the sixth planet from the sun since 

2004.

The probe continues to send back stunning images as well as important data. In this photo, a softly hued Saturn is embraced by the shadows of its rings. The gas planet's subtle northward gradation from gold to azure is a striking visual effect that scientists don't fully understand. A current theory is that it may be related to seasonal influences, tied to the cold temperatures in the northern (winter) hemisphere. Images taken with blue, green and red spectral filters were used to create this color view, which approximates the scene as it would appear to the human eye. Credit: NASA/JPL-Caltech/Space Science Institute Saturn's rings It's difficult to get a sense of scale when viewing Saturn's rings, but the Cassini Division (seen here between the bright B ring and dimmer A ring) is almost as wide as the planet Mercury. The 2,980-mile-wide (4,800-kilometer-wide) division in Saturn's rings is thought to be caused by the moon Mimas.

Credit: NASA/JPL-Caltech/Space Science Institute Saturn's moons Three of Saturn's moons -- Tethys, Enceladus and Mimas -- can be seen Cassini spacecraft photo released by NASA, February 22, 2016. Tethys (660 miles or 1,062 kilometers across) appears above the rings, while Enceladus (313 miles or 504 kilometers across) sits just below center. Mimas (246 miles or 396 kilometers across) hangs below and to the left of Enceladus. Credit: NASA/JPL-Caltech/Space Science Institute Saturn's rings From afar, Saturn's rings look like a solid, homogenous disk of material, in this image from January 8, 2015. But upon closer examination from Cassini, the varied structures in the rings at almost every scale imaginable are apparent. Structures in the rings can be caused by many things, but often times Saturn's many moons are the culprits.

Credit: NASA/JPL-Caltech/Space Science Institute Saturn and Dione This image was taken in a wavelength that is absorbed by methane, May 27, 2015. Dark areas seen here on Saturn are regions with thicker clouds, where light has to travel through more methane on its way into and back out of the atmosphere. Since Dione (698 miles or 1,123 kilometers across) doesn't have an atmosphere rich in methane the way Saturn does, it does not experience similar absorption -- the sunlight simply bounces off its icy surface. Credit: NASA/JPL-Caltech/Space Science Institute Enceladus's water vapor jets, emitted from the southern polar region. Credit: NASA/JPL-Caltech and Space Science Institute Tethys and Rhea moons Tethys appears to be peeking out from behind Rhea, April 20, 2012. Credit: NASA/JPL-Caltech/Space Science Institute Saturn's moon Enceladus The tortured surface of Saturn's moon Enceladus and its fascinating ongoing geologic activity tell the story of the ancient and present struggles of one tiny world.

The enhanced color view of Enceladus seen here is largely of the southern hemisphere and includes the south polar terrain at the bottom of the image. The image mosaic was created from 21 false-color frames taken on March 9 and July 14, 2005. Credit: NASA/JPL-Caltech/Space Science Institute Three moons The three moons shown here -- Titan (3,200 miles or 5,150 kilometers across), Mimas (246 miles or 396 kilometers across), and Rhea (949 miles or 1,527 kilometers across) -- show marked contrasts, March 25, 2015. Titan, the largest moon in this image, appears fuzzy because only its cloud layers are seen. And because Titan's atmosphere refracts light around the moon, its crescent "wraps" just a little further around the moon than it would on an airless body. Rhea (upper left) appears rough because its icy surface is heavily cratered. And a close inspection of Mimas (center bottom), though difficult to see at this scale, shows surface irregularities due to its own violent history.

Credit: NASA/JPL-Caltech/Space Science Institute Saturn's auroral emissions The Cassini spacecraft obtained these images of Saturn's auroral emissions, which are similar to Earth's Northern Lights, June 21, 2005. These are the first images from the mission to capture the entire "oval" of the auroral emissions at Saturn's south pole. In the side-by-side, false-color images, blue represents aurora emissions from hydrogen gas, while red-orange represents reflected sunlight. The photos show that the aurora lights at the polar regions respond rapidly to changes in the solar wind. Credit: NASA/JPL-Caltech/Space Science Institute Hyperion A stunning false-color view of Saturn's moon Hyperion reveals crisp details across the strange, tumbling moon's surface. Differences in color could represent differences in the composition of surface materials. The view was obtained during Cassini's close flyby on September 26, 2005. Hyperion has a notably reddish tint when viewed in natural color. The red color was toned down in this false-color view, and the other hues were enhanced, in order to make more subtle color variations across Hyperion's surface more apparent.

Credit: NASA/JPL-Caltech/Space Science Institute Tethys and Mimas Saturn is circled by its rings (seen nearly edge-on in this image), as well as by the moons Tethys (the large bright body near the lower right hand corner of this image) and Mimas (seen as a slight crescent against Saturn's disk above the rings, at about 4 o'clock). The shadows of the rings, each ringlet delicately recorded across Saturn's face, also circle around Saturn's south pole. The image was taken in visible light with the Cassini spacecraft wide-angle camera on August 14, 2014. Credit: NASA/JPL-Caltech/Space Science Institute Saturn's Tethys Like a drop of dew hanging on a leaf, Tethys appears to be stuck to the A and F rings from this perspective, July 14, 2014. Tethys (660 miles, or 1,062 kilometers across) is composed primarily of ice.

Credit: NASA/JPL-Caltech/Space Science Institute Saturn's moon Iapetus This near-true color view from Cassini reveals the colorful and intriguing surface of Saturn's moon Iapetus in unrivaled clarity, January 7, 2005. Credit: NASA/JPL-Caltech/Space Science Institute Saturn's Phoebe During its historic close encounter with Phoebe, the Cassini spacecraft captured a series of high resolution images of the small moon, six of which have been put together to create this mosaic, June 23, 2004. Credit: NASA/JPL-Caltech/Space Science Institute Saturn's Titan This specially processed composite view reveals a tremendous amount of structure in the northern polar atmosphere of Titan. The hazes in Titan's atmosphere are known to extend hundreds of kilometers above the surface. Ten images taken on January 18, 2006, during a brief period, were processed to enhance fine detail and then were combined to create this view.

Credit: NASA/JPL-Caltech/Space Science Institute Saturn's Titan A mosaic of nine processed images acquired during Cassini's first very close flyby of Saturn's moon Titan on October 26, 2004, constitutes the most detailed full-disc view of the mysterious moon. The images that comprise the mosaic have been processed to reduce the effects of the atmosphere and to sharpen surface features. Credit: NASA/JPL-Caltech/Space Science Institute Wide-angle view of Saturn The globe of Saturn, seen here in natural color, is reminiscent of a holiday ornament in this wide-angle view from NASA's Cassini spacecraft, July 22, 2013. The characteristic hexagonal shape of Saturn's northern jet stream, somewhat yellow here, is visible. At the pole lies a Saturnian version of a high-speed hurricane, eye and all. Credit: NASA/JPL-Caltech/Space Science Institute Titan and Rhea Saturn's largest and second largest moons, Titan and Rhea, appear to be stacked on top of each other in this true-color scene from NASA's Cassini spacecraft, on June 16, 2011.

Credit: NASA/JPL-Caltech/Space Science Institute Saturn's southern hemisphere With winter approaching in the southern hemisphere of Saturn comes the familiar blue hue of the planet's cold season, July 29, 2013. Credit: NASA/JPL-Caltech/Space Science Institute Saturn and Titan Slipping into shadow, the south polar vortex at Saturn's moon Titan still stands out against the orange and blue haze layers that are characteristic of Titan's atmosphere on July 30, 2013. Credit: NASA/JPL-Caltech/Space Science Institute Darkside of Saturn's Titan NASA's Cassini spacecraft looks towards the dark side of Saturn's largest moon, Titan, capturing the blue halo caused by a haze layer that hovers high in the moon's atmosphere on November 3, 2013. Credit: NASA/JPL-Caltech/Space Science Institute Titan Using a special spectral filter, the high-resolution camera aboard NASA's Cassini spacecraft was able to peer through the hazy atmosphere of Saturn's moon Titan on October 7, 2013.

Credit: NASA/JPL-Caltech/Space Science Institute Saturn's rings A dynamical interplay between Saturn's largest moon, Titan, and its rings is captured in this view from NASA's Cassini spacecraft on September 20, 2009. Credit: NASA/JPL-Caltech/Space Science Institute Icy Enceladus Saturn's moon Enceladus, covered in snow and ice, resembles a perfectly packed snowball in this image from NASA's Cassini mission on March 10, 2012. The image was taken with the Cassini spacecraft narrow-angle camera using filters sensitive to ultraviolet, visible and infrared light. Credit: NASA/JPL-Caltech/Space Science Institute Enceladus NASA's Cassini captures a still and partially sunlit Enceladus on April 7, 2010, using filters sensitive to ultraviolet, visible and infrared light. Credit: NASA/JPL-Caltech/Space Science Institute Saturn's Janus Saturn's equinox is captured here in a mosaic of light and dark, September 21, 2009. The moon Janus (179 kilometers, 111 miles across) is on the lower left of this image.

Epimetheus (113 kilometers, 70 miles across) appears near the middle bottom. Pandora (81 kilometers, 50 miles across) orbits outside the rings on the right of the image. The small moon Atlas (30 kilometers, 19 miles across) orbits inside the thin F ring on the right of the image. Credit: NASA/JPL-Caltech/Space Science Institute Tethys: Saturn's Icy Moon Tethys: Saturn's Icy Moon By Nola Taylor Redd, Space.com Contributor | June 29, 2016 08:48pm ET MORE This raw, unprocessed image was taken by NASA's Cassini spacecraft on May 20, 2012. The camera was pointing toward the Saturn moon Tethys at approximately 81,580 miles (131,290 kilometers) away. Credit: NASA/JPL/Space Science Institute Although Saturn's moon Tethys appears bland and neutral from space, it contains a number of features of interest. A large impact basin and an enormous trench are just two of the intriguing landmarks that mar the surface of the icy moon.

A heat map of the satellite also reveals a configuration that resembles the title character in the video game Pac-Man. One of the great mysteries of Tethys is where large arcs of red material came from . The arcs were first detected in 2004, but really became prominent in 2015 images of the moon from the Cassini spacecraft taken in better lighting conditions, during summertime in that region. Because they lie across other features of Tethys, they are relatively young. Red is an unusual color in the solar system (only seen in patches on the moons Dione and Europa, for example). The team suggested the red might be due to chemical impurities, remainders of gas from the moon's interior, or features that could not be resolved in the image. [ Photos: The Rings and Moons of Saturn ] Discovery and naming Tethys was one of a pair of moons orbiting Saturn that was discovered by Giovanni Cassini in 1684. The Italian astronomer spotted both Tethys and Dione on March 21, making them the last two of the four moons first found by the astronomer (the other two were Iapetus and Rhea ).

Cassini's discoveries were the first found in orbit around the ringed planet. Cassini called his moons the Sidera Lodoicea — the Stars of Louis — after King Louis XIV. For almost 200 years, the moons of Saturn were referred to numerically, based on their distance from the planet. Tethys, which orbits 183,100 miles (294,660 kilometers) out, was termed Saturn III, since it is the third most distant of the major satellites. In 1847, British astronomer John Herschel suggested that the moons surrounding the ringed planet should take their names from the Titans, the mythological siblings of the Greek god Cronus (Saturn to the Romans). In Greek mythology, Tethys was the daughter of Uranus and Gaea, and the wife of Oceanus. A watery goddess, she was considered mother of the chief rivers, and birthed 3,000 daughters known as the Oceanids. During the war between the Titans and the Olympians, Tethys raised Hera, wife of Zeus.

Features on Tethys are taken from Homer's poem "The Odyssey." A frozen surface Although Herschel didn't realize it at the time, a watery goddess made an ideal name for the moon. Like many of the satellites around Saturn, Tethys is made up almost entirely of water-ice. At average temperatures of minus 187 degrees C (minus 305 degrees F), the ice on the lunar surface responds much like rock. The photo of Saturn's moon Tethys was taken on April 14, 2012 and received on Earth April 15, 2012. The camera was pointing toward Tethys at approximately 185 kilometers away, and the image was taken using the CL1 and CL2 filters. This image has not been validated or calibrated. Credit: NASA/JPL/Space Science Institute. The frigid surface is highly reflective, a characteristic only increased by the bombardment of water-ice particles from the plumes of its sister moon Enceladus . The floor of many of the numerous craters on the satellite shine brightly, further suggesting the presence of water-ice.

As the third closest of the major moons, Tethys feels a strong pull from Saturn that could have kept it partially molten longer than it would have suffered at a more distant orbit. As a result, it lacks the heavy cratering of Rhea or Dione, which suggests that it was resurfaced in the recent past. Tethys has no atmosphere, and keeps one face perpetually pointed toward its parent planet. It travels around Saturn once every 1.888 Earth days. Trenches and craters Although most of signs of impact on Tethys are relatively small, a giant one known as the Odysseus Crater covers nearly two-fifths of the moon's 662-mile (1,066 km) diameter. At 250 miles (400 km) across, the crater is about the size of Saturn's smallest major moon, Mimas . An ancient feature, the crater is nearly flat, with its floor conforming to the shape of the moon. The enormous impact should have shattered a stiff, solid body. Tethys obviously remains intact, leading scientists to conclude that the crash occurred early in the moon's history, when Tethys was still partially molten.

A large trench runs from the moon's north pole to its south pole, extending 1,200 miles (2,000 km) and spanning nearly three-quarters of the planet's circumference. Known as the Ithaca Chasma, after the country ruled by Odysseus, the trench is 62 miles (100 km) wide and 2 to 3 miles (3 to 5 km) deep. The chasm could have been created by the same impact that formed the Odysseus crater, which lies on the other side of the moon. Another possible explanation is that the surface froze before the interior, leading to an expansion that cracked the crust. This view of the Pac-Man shapes on Saturn's moons Mimas and Tethys show variations in the heat signature of both moons as seen by an infrared-detecting tool on NASA's Cassini spacecraft. Credit: NASA/JPL-Caltech/GSFC/SWRI While taking thermal data, NASA's Cassini spacecraft revealed a Pac-Man-like shape on the icy satellite. The video game character configuration may have been created by the varying surfaces on the moon.

Scientists think that electrons bombarding the leading face of the satellite could have turned the region into hard-packed ice that changes temperature more slowly than the rest of the moon. Tethys' Pac-Man heat map was the second to be found in the solar system, only two years after Cassini noticed a similar configuration on Mimas. Other observations of Tethys suggest that its crust is similar to Dione and Rhea, two other icy moons of Saturn. A 2016 study in the journal Icarus noted that Ithaca Chasma was not caused by the small body that created the crater Odysseus, contrary to what was believed in the past. However, the "Odysseus impact event" could have created some bands of large ice particles seen stretching across the north and south. Separately, a 2016 presentation at the American Astronomical Society suggested some craters on Tethys (and other moons) could have been from leftover debris from a previous generation of moons.

Quick facts Average orbit around Saturn: 183,100 miles (294,672 km) Closest approach: 183,083 miles (294,643 km) Farthest approach: 183,119 (294,701 km) Orbit eccentricity: 0.0001 The moon Rhea - History, structure, surface, atmosphere, ring system The moon Rhea Rhea. © based on NASA images Rhea, second largest of Saturn’s natural satellites or moons, was discovered in the year 1672 by Giovanni Domenico Cassini, an Italian-French astronomer. Giovanni Domenico Cassini. © National Portrait Gallery, London It is also the ninth biggest moon in the existing solar system in terms of size. The moon got its name from the Greek mythological character Rhea who is the revered mother of all the gods. Also given the name Saturn V, it is the fifth moon that moves outward from the planet.

The four moons of Saturn that Cassini discovered were named “Stars of Louis” (Tethys, Lapetus, Rhea, and Dione) to honor the French king. Most astronomers, however, referred to these moons (and Titan) as Saturn I through V. With the discovery of two more moons in 1789 (Mimas and Enceladus), the names extended to Saturn VII. The names of the seven moons of Saturn were named after Greek mythological Titans (brothers and sisters of Cronos) by John Herschel who was the son of William Herschel, the discoverer of Uranus. Physical structure and characteristics of Rhea Rhea’s density of 1.236 g/cm3 indicates that it is primarily composed of water ice constituting about 75 percent and rocky material makes up for the balance 25 percent. Though it is ninth largest in size, it holds the tenth position in terms of mass. You can see Rhea on the edge of Saturn. © NASA Rhea has an icy body and was initially assumed to have a core made of rocks. However, research by various astronomers has only proved controversial.

Whereas some of them argued that the moment of inertia reading suggested a rocky core, others have pointed out to a homogenous interior. The scientific community points out to the fact that additional measurements may be required to resolve the problem and come to a conclusion. The shape of Rhea suggests that it is in hydrostatic equilibrium and has a homogenous body. Rhea is about 763 km in radius and has a surface area of 7.3 million km2. Rhea’s distance from Saturn is about 527,040 km and it takes the same time to rotate on its axis as well as revolve around Saturn (4.516 Earth days). Its orbit is almost a circle and it has a surface gravity of about 0.265 m/s2. Scientific models that have been created to point out that Rhea may be capable of sustaining a liquid ocean of water inside through the heating produced by radioactive decay. One side of Rhea is always facing Saturn. Surface and geological characteristics of Rhea Thanks to Cassini space probe we have picture of Rhea’s surface. © NASA Rhea’s surface features very closely resemble that of another moon Dione and therefore suggest similar histories of formation.

Its leading and trailing hemispheres (the hemisphere that faces the direction opposite to the orbital movement) have different features that bear close semblance to those of Dione. Under direct light of the sun Rhea’s temperature is about 174 degrees centigrade and under shade it is between -200 and -220 degrees centigrade. Rhea’s leading hemisphere is very reflective. The surface of Rhea has several craters like that of our moon and its trailing hemispheres are covered with a wispy terrain very similar to that found in Dione. These appear as brighter streaks in the darker trailing hemisphere. Deposits at its equatorial region due to material that has deorbited from its rings region show up as a line on Rhea. The craters of Rhea are not walled high nor do they have a peak towering their centers. Two large impact craters dot the trailing hemisphere of Rhea and are approximately 400 and 500 km across. The impact crater Tirawa lies more to the north and is comparable in structure to Odysseus basin on the moon Tethys. The other impact crater is called Mamaldi.

Inktomi also nicknamed as ‘Splat’ is prominent and one of the youngest craters of Rhea with a diameter of 48 km. Internal structure of Rhea. © NASA images Craters on Rhea’s surface. © NASA The crater densities are also starkly different on the surface of Rhea. This has given rise to two different geologically characterized areas: one that contains craters larger than 40 km in diameter and the second containing craters smaller in size. The second is found in polar and equatorial regions of Rhea. The wispy terrain on the trailing hemisphere (that is bright and prominent) is thought to have been caused by ejections from ice volcanoes when Rhea was just forming and its interiors were filled with liquid. The bright streaks extend for hundreds of kilometers across the terrain. Some of the canyons are several hundred meters high. The walls of the canyon appear bright when dark material falls off and exposes fresh water ice. The brighter streaks in Dione’s darker trailing hemisphere has shown the existence of ice cliffs formed due to extensive fracturing of the surface of that moon.

Rhea’s atmosphere In the year 2010, the space probe called Cassini discovered that Rhea has a very thin atmosphere made up of carbon dioxide and oxygen existing in an approximate proportion of 2 to 5. This is the first time that a spacecraft has captured an atmosphere of oxygen other than in Earth. The source of oxygen is purported to be the dissociation of water ice molecules at the surface through ions created by the magnetic cavity of Saturn. In simple words, the oxygen arises when Saturn’s magnetic field rotates over the moon Rhea. The source of carbon dioxide is not very clear. It is thought to come from the organics present in the water ice or it may have occurred due to the release of the gas which was dissolved, frozen, or absorbed in other material. Rhea’s ring system Cassini space probe. © NASA There were initial suggestions about the existence of rings around Rhea, as announced by NASA in 2008. Rings were thought to exist because of a changed pattern of the flow of electrons that was observed. It was thought that dust and debris were denser near the moon forming three rings of high density.

However, later observations by the space probe Cassini in 2010 did not reveal any material and made way for further conclusive evidence to be found to establish the presence of rings. Rhea’s first images were got through pictures taken by Voyager I and II in the years 1980 and 1981. The Cassini orbiter has been able to procure images of Rhea after its four fly-bys in the years 2005, 2007, 2010 and 2011. The last targeted fly-by by the orbiter was as close as 69 km from the moon. Many additional images from long to moderate distances shot by the orbiter also exist. Saturn's moon Rhea Saturn's moon Rhea October 26, 2015 by [Vid=35206], Universe Today Saturn’s moon Rhea, as imaged by the Cassini-Huygens spacecraft. Credit: NASA/JPL-Caltech The Cronian system (i.e. Saturn and its system of rings and moons) is breathtaking to behold and intriguing to study.

Besides its vast and beautiful ring system, it also has the second-most satellites of any planet in the Solar System. In fact, Saturn has an estimated 150 moons and moonlets – and only 53 of them have been officially named – which makes it second only to Jupiter. For the most part, these moons are small, icy bodies that are believed to house interior oceans. And in all cases, particularly Rhea, their interesting appearances and compositions make them a prime target for scientific research. In addition to being able to tell us much about the Cronian system and its formation, moons like Rhea can also tell us much about the history of our Solar System. Discovery and Naming: Rhea was discovered by Italian astronomer Giovanni Domenico Cassini on December 23rd, 1672. Together with the moons of Iapetus, Tethys and Dione, which he discovered between 1671 and 1672, he named them all Sidera Lodoicea ("the stars of Louis") in honor of his patron, King Louis XIV of France. However, these names were not widely recognized outside of France.

In 1847, John Herschel (the son of famed astronomer William Herschel, who discovered Uranus, Enceladus and Mimas) suggested the name Rhea – which first appeared in his treatise Results of Astronomical Observations made at the Cape of Good Hope. Like all the other Cronian satellites, Rhea was named after a Titan from Greek mythology, the "mother of the gods" and one the sisters of Cronos (Saturn, in Roman mythology). Size, Mass and Orbit: With a mean radius of 763.8±1.0 km and a mass of 2.3065 ×1021 kg, Rhea is equivalent in size to 0.1199 Earths (and 0.44 Moons), and about 0.00039 times as massive (or 0.03139 Moons). It orbits Saturn at an average distance (semi-major axis) of 527,108 km, which places it outside the orbits of Dione and Tethys, and has a nearly circular orbit with a very minor eccentricity (0.001).

With an orbital velocity of about 30,541 km/h, Rhea takes approximately 4.518 days to complete a single orbit of its parent planet. Like many of Saturn's moons, its rotational period is synchronous with its orbit, meaning that the same face is always pointed towards it. Composition and Surface Features: The moons of Saturn, from left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan (background), Iapetus (top), and Hyperion (bottom). Credit: NASA/JPL/Space Science Institute With a mean density of about 1.236 g/cm3, Rhea is estimated to be composed of 75% water ice (with a density of roughly 0.93 g/cm³) and 25% of silicate rock (with a density of around 3.25 g/cm³). This low density means that although Rhea is the ninth-largest moon in the Solar System, it is also the tenth-most massive. In terms of its interior, Rhea was originally suspected of being differentiated between a rocky core and an icy mantle.

However, more recent measurements would seem to indicate that Rhea is either only partly differentiated, or has a homogeneous interior – likely consisting of both silicate rock and ice together (similar to Jupiter's moon Callisto). Models of Rhea's interior also suggest that it may have an internal liquid-water ocean, similar to Enceladus and Tethys. This liquid-water ocean, should it exist, would likely be located at the core-mantle boundary, and would be sustained by the heating caused by from decay of radioactive elements in its core. Rhea's surface features resemble those of Dione, with dissimilar appearances existing between their leading and trailing hemispheres – which suggests that the two moons have similar compositions and histories. Images taken of the surface have led astronomers to divide it into two regions – the heavily cratered and bright terrain, where craters are larger than 40 km (25 miles) in diameter; and the polar and equatorial regions where craters are noticeably smaller. Another difference between Rhea's leading and trailing hemisphere is their coloration. The leading hemisphere is heavily cratered and uniformly bright while the trailing hemisphere has networks of bright swaths on a dark background and few visible craters.

It had been thought that these bright areas (aka. wispy terrain) might be material ejected from ice volcanoes early in Rhea's history when its interior was still liquid. However, observations of Dione, which has an even darker trailing hemisphere and similar but more prominent bright streaks, has cast this into doubt. It is now believed that the wispy terrain are tectonically-formed ice cliffs (chasmata) which resulted from extensive fracturing of the moon's surface. Rhea also has a very faint "line" of material at its equator which was thought to be deposited by material deorbiting from its rings (see below). Rhea has two particularly large impact basins, both of which are situated on Rhea's anti-Cronian side (aka. the side facing away from Saturn). These are known as Tirawa and Mamaldi basins, which measure roughly 360 and 500 km (223.69 and 310.68 mi) across.

The more northerly and less degraded basin of Tirawa overlaps Mamaldi – which lies to its southwest – and is roughly comparable to the Odysseus crater on Tethys (which gives it its "Death-Star" appearance). Atmosphere: Views of Saturn’s moon Rhea, with false-color image showing elevation data at the right. Credit: NASA/JPL/Space Science Institute Rhea has a tenuous atmosphere (exosphere) which consists of oxygen and carbon dioxide, which exists in a 5:2 ratio. The surface density of the exosphere is from 105 to 106 molecules per cubic centimeter, depending on local temperature. Surface temperatures on Rhea average 99 K (-174 °C/-281.2 °F) in direct sunlight, and between 73 K (-200 °C/-328 °F) and 53 K (-220 °C/-364 °F) when sunlight is absent. The oxygen in the atmosphere is created by the interaction of surface water ice and ions supplied from Saturn's magnetosphere (aka. radiolysis).

These ions cause the water ice to break down into oxygen gas (O²) and elemental hydrogen (H), the former of which is retained while the latter escapes into space. The source of the carbon dioxide is less clear, and could be either the result of organics in the surface ice being oxidized, or from outgassing from the moon's interior. Rhea may also have a tenuous ring system , which was inferred based on observed changes in the flow of electrons trapped by Saturn's magnetic field. The existence of a ring system was temporarily bolstered by the discovered presence of a set of small ultraviolet-bright spots distributed along Rhea's equator (which were interpreted as the impact points of deorbiting ring material). However, more recent observations made by the Cassini probe have cast doubt on this. After taking images of the planet from multiple angles, no evidence of ring material was found, suggesting that there must be another cause for the observed electron flow and UV bright spots on Rhea's equator. If such a ring system were to exist, it would be the first instance where a ring system was found orbiting a moon.

Hemispheric color differences on Saturn’s moon Rhea are apparent in this false-color view of the anti-Cronian side, from NASA’s Cassini spacecraft. Credit: NASA/JPL/SSI Exploration: The first images of Rhea were obtained by the Voyager 1 and 2 spacecraft while they studied the Cronian system, in 1980 and 1981, respectively. No subsequent missions were made until the arrival of the Cassini orbiter in 2005. After it's arrival in the Cronian system, the orbiter made five close targeted fly-bys and took many images of Saturn from long to moderate distances. The Cronian system is definitely a fascinating place, and we've really only begun to scratch its surface in recent years. In time, more orbiters and perhaps landers will be traveling to the system, seeking to learn more about Saturn's moons and what exists beneath their icy surfaces. One can only hope that any such mission includes a closer look at Rhea, and the other "Death Star Moon", Dione. Saturn’s second-largest moon Rhea, pictured by the Cassini probe on March 29, 2012.

Credit: NASA/JPL The video will load shortly Rhea Moon | PlanetsEdu.com Titan Rhea Rhea is one of Saturn’s moons. It was discovered in 1672 by the Italian astronomer Giovanni Domenico Cassini. Diameter: 1.53 x 106 m Mass: 2.31 x 1021 kg Composition: Water-ice with ammonia Orbit: 5.27 x 105m from Saturn Rhea is the second largest of Saturn’s moons and the ninth-largest moon in the Solar System. Saturn has 62 moons with confirmed orbits, 53 of which have names. 24 of these are regular satellites that orbit Saturn with only small inclinations to the planet’s equatorial plane. 37 of the remaining 38 are irregular satellites that orbit at high inclinations and a much greater distance from Saturn. These are probably minor planets captured by Saturn’s gravity or debris from their break-up. The final irregular satellite is unclear. Saturn is the Roman god of agriculture and harvest. The first of the plant’s moons to be discovered were named after his brothers and sisters.

When all of these names had been used, other ones were taken from additional characters in Greek or Roman mythology or from the mythologies of other cultures. The irregular satellites are named after figures from Inuit, Gallic and Norse mythology. Rhea is named after the Titaness Rhea from Greek mythology. She was the daughter of Uranus and Gaia and known as the “mother of the gods”. The name was first suggested 1847 by William Herschel’s son John. Rhea is also designated Saturn V as the fifth moon going outward from the planet. Rhea is predominantly a water-ice body. There is a large amount of ammonia in the ice. The presence of a rock core in Rhea was discounted after the Cassini mission in 2004. Instead, astronomers proposed that Rhea condensed from a concentric family of gas rings that had been cast off from a proto-Saturnian cloud. Rhea’s surface is old and heavily cratered, like Jupiter’s moon, Callisto. Its brightness varies with its position relative to Saturn.

Rhea has two large basins caused by impacts on the anti-Cronian (facing away from Saturn) hemisphere with diameters between 400 and 500km. This hemisphere shows tectonic fractures similar to those on Dione that have exposures of bright ice on escarpment slopes. Impact craters in Rhea’s medium-latitude regions have diameters larger than 40 km, while those in polar or equatorial regions are usually smaller. A tenuous oxygen and carbon dioxide atmosphere around Rhea was discovered by NASA in 2010. As with most of Saturn’s other moons, Rhea rotates synchronously around the planet and keeps the same hemisphere towards Saturn and the same hemisphere facing forward during the orbit. Rhea takes 4.52 Earth days to rotate around Saturn and to complete one spin on its own axis. Along with Saturn and its other satellites, it takes 29.4 Earth years to orbit the sun. Spacecraft missions Voyager 1 and 2 in 1980 and 1981 provided the first images of Rhea. The Cassini spacecraft mission to Saturn sent back detailed images of the moon.

The closest Cassini flyby was at 69 km in January 2011. Image processing was able to produce details down to a few metres on size. Share this with the Universe! Rhea Gallery A visible light image of Rhea. The image was taken by Cassini; the spacecraft was closer to Rhea than Rhea was to Saturn A false colour image of the side of Rhea that always faces Saturn An image of the side of Rhea that always faces Saturn taken by Cassini in 2010 Rhea, Enceladus, Mimas, and Tethys, oh my! | The Planetary Society The Planetary Society Join our eNewsletter for updates & action alerts Spam Control Text: Please leave this field empty Blogs Topics: Enceladus , pretty pictures , Tethys , Cassini , amateur image processing , Rhea , global views , Saturn's moons , Mimas For a very long time Cassini has been in an orbit that carried it high above and below the plane of the rings.

This has been fantastic for observations of the rings and planet during the equinox season, and has permitted some cool observations of Titan's poles (since Cassini always comes back to Titan sooner or later for an orbit-adjusting gravity-assist flyby). But it's meant there's been very little in the way of high-resolution views of the other moons. With the last Titan flyby on October 12, Cassini came back to an orbit that's nearly in the equatorial plane, and immediately rewarded us with some fine views of several of the icy moons. Here are a bunch of images of those moons taken over the 13th and 14th: Rhea, Saturn's second-largest moon. Cassini got quite close so used the wide-angle rather than narrow-angle camera to get a whole-moon view: NASA / JPL-Caltech / SSI Rhea Cassini observed a slightly less than half-full Rhea on October 13, 2009. Next came Enceladus. This view is particularly nice.

It's really unusual to be able to see Enceladus' surface and its geysers clearly in a single shot, because the geysers are most visible when they're lit from behind, leaving Enceladus only a skinny crescent; and they have to use a long enough exposure that any sunlit surface is overexposed. That's the geometry here, but Enceladus' night-side is half-lit by Saturnshine, which lets us take in sunlit crescent, nightside surface, and geysers all in a single exposure. NASA / JPL-Caltech / SSI Enceladus Cassini captured this lovely view of a crescent Enceladus, lit also by Saturnshine, with its south polar geysers backlit by the Sun, on October 14, 2009. Here's a neat view of Mimas that makes its egg shape obvious. Its funny shape arises from the balance between self-gravity (which would make it spherical) and rotation (which causes it to bulge in the middle) and tides from Saturn (which stretches it out in the directions pointing toward and away from Saturn).

All of Saturn's moons are triaxial ellipsoids, but Mimas is small enough that its self-gravity is weak enough that the deviations from the self-gravitational spherical shape are really obvious. NASA / JPL-Caltech / SSI Mimas Cassini shot a nearly-full-phase view of Mimas on October 14, 2009. From this perspective, Mimas' flattened ellipsoidal shape is evident. Finally, as Cassini sped away from its encounters with these inner moons, it got nice long views of a fully lit Tethys. This very nice color composite arrived in my mailbox from Sean Walker, an editor at Sky and Telescope magazine. NOTE: Since I first posted this I have rotated the image 180 degrees so that north is up. NASA / JPL-Caltech / SSI / color composite by Sean Walker Tethys in enhanced color Cassini captured this view of Tethys at nearly full phase on October 14, 2009. A composite of images captured through infrared, green, and ultraviolet filters enhances the color variations visible across Tethys' surface.

It's hard to get a handle on the orbital geometry that sets up the possibilities of these views, and also on why the different moons appear at different phases to Cassini. So for fun I thought I'd put together an animation that shows how Cassini's orbital motion relates to the orbital motions of the inner moons. Below that are four panels that show you approximately the field of view of Cassini's narrow-angle camera, pointed at the four moons that Cassini photographed over this period. Enjoy! NASA / JPL-Caltech / Solar System Simulator (David Seal) / animation by Emily Lakdawalla 24 hours of Cassini orbiting among Saturn's moons On October 13 and 14, 2009, Cassini had returned to a trajectory that was close to the plane of Saturn's rings, allowing it to have numerous "Voyager-class" encounters with Saturn's inner icy moons. This animation covers one full day of the Cassini mission and shows how its orbit relates to the motions of the moons (top panel) and how its narrow-angle camera view of four of those moons -- Rhea, Mimas, Enceladus, and Tethys -- changed with the changing positions of spacecraft and moons.

Question: Rhea and Tethys are moons of which planet?
Answer: SATURN